There has been developed a monitoring system that detects a monitoring event from an image taken by a camera, and records the image from the time of detection as a trigger (for example, Patent Document 1: “Image Storage Device, Monitoring System, and Storage Medium”). Image recording start is triggered, for example, when a person's face which has been photographed matches face information registered in advance as a result of comparison process the photographed face and the face information. In Patent Document 1, “when the number of areas determined as a flesh color in an input frame image exceeds a given number” is one trigger of the image recording start.